For these reasons, in addition to forced air circulation produced by an integrated ventilation system, by which ambient or cooled air can travel across free spaces between the boards, heat-exchange means have been provided specifically for each of the highly dissipating components, and notably heat pipes, which make it possible to carry the heat given off by a component, out of the module (computer) to a heat exchanger, such as for example an external radiator or an external cold plate.
A heat pipe is a passive device for carrying heat, based on the liquid-vapour change of state of a fluid. It is a sealed enclosure, such as an aluminium or copper tube, which contains a liquid in balance with its vapour phase. Typically, one end is in thermal contact with the component which gives off the heat, for example a processor: this is the hot object. The other end is in thermal contact with the heat exchanger: the cold object. Under the effect of the operation of the processor, the walls of the tube heat up and vaporize the liquid. The pressure of the vapour increases and causes the vapour to migrate to the cold object. Under the cold object, the relative cooling of the walls causes the vapour to condense, increasing the volume of the liquid which migrates and flows back to the hot object.
Each end of a heat pipe is therefore furnished with a shoe providing a placement surface which will allow a good thermal contact with the hot and cold objects.
In practice, the heat pipe is thermally efficient at cooling a dissipative component provided that it is mounted suitably on this component, in good thermal contact with it, that is to say putting up a weak thermal resistance to the passage of the calories, typically a thermal resistance of less than one degree per watt.
If it is defectively mounted, the cooling will not only be ineffective but it may also cause a deterioration of the operation of the component and even its destruction, by thermal stress, when the computer is powered up by not effectively countering the thermal drift of the active portions of the component. This defective mounting may in practice occur in the manufacture of the computer, or during the repair of the computer.
When a visual inspection of the mounting between the heat pipe and the component is possible, it does not usually make it possible to guarantee the quality of the thermal contact, which may be adversely affected by the state of the surfaces in contact being polluted by dust or fluids.